The form of a text is the type of text you are reading or watching. The form of Macbeth is a dramatic play. More specifically, it is a tragedy. The simplest definition of a tragedy would be "a play with an unhappy ending". While this is true, it is probably just a bit too simple.
Tragic plays can be traced back all the way to the beginnings of drama in Ancient Greece. In Poetics, one of the first books of literary criticism, Aristotle set out the key features of tragedy. He stated that tragic plays would involve a protagonist (the leading central figure) who is usually of royal or noble birth. In the course of the play, the protagonist reveals a fatal flaw (a character defect) which causes him or her to go from success and happiness to failure, misery and, often, death at the hands of an antagonist (his opposite). Tragedy set out to stir up feelings of fear and pity in the audience – this is known as catharsis.
All of these things can be seen at work in Macbeth. The protagonist is clearly Macbeth himself, a thane of the Scottish nobility. His fatal flaw is his ambition and this drives the action forward. Macbeth is basically a good man who goes wrong. He is driven by a need for power which eventually sets him on a path to his own destruction. His wife shares this fatal flaw with him.
While Macbeth clearly achieves his ambition to become king, it is at the expense of his happiness. He feels he needs to murder, lie and behave brutally to others in order to keep his power. Eventually he goes too far when he slaughters Macduff's family. This causes Macduff to take up a position as the play's antagonist – Macbeth's opposite. Eventually Macduff kills Macbeth in face-to-face combat. While exciting to watch, all of this should cause feelings of horror and regret in the audience.